Increased dynamic stiffness across the frequency range from 35 to 0.5 Hz in maximally activated strips of aorta obtained from senescent beagles as compared to adults was demonstrated. These changes accompany changes in intrinsic impedance measured in the same animals prior to sacrifice. Dynamic stiffness measured during a cardiac contraction in rat cardiac muscle has been shown to be dependent on the degree of activation produced by adjustment of bathing (Ca++), and implies that stiffness measured thusly is in part directly related to the activation process.